The threaded fixation element, such as a threaded rod or bone nail, and the fluted fixation element, such as a fixation plate or lateral fixation block, of the conventional bone fixing device are generally defective in design in that they do not meet to form exactly an angle of 90 degrees, and that the nut cannot be fastened securely with the threaded fixation element. With a view to overcoming such shortcomings as described above, the U.S. Pat. Nos. 4,611,581 and 4,696,290 disclosed respectively a bone fixing plate which is provided with scallops intended to enhance the effect of fastening the threaded fixation element with the fluted fixation element of the conventional bone fixing device. However, the bone fixing plates disclosed in the above-mentioned U.S. patents fail to provide an effective remedial measure. As a result, an improved system was disclosed by Hiromi Matsuzaki, et al. in SPINE, VOLUME 15, NUMBER 11 (1990), pp. 1159-1164. The improved system comprises a fluted fixation element which is provided respectively on both faces of the slot or the through hole thereof with a recessed arcuate surface capable of cooperating with a protruding arcuate surface of an arresting portion of the fixation element having the threaded portion and the arcuate protruding surface of the fixation nut. In view of the fact that the centers of the circles of these two arcuate recessed surfaces are fixed, the fixation element is located on a line connecting these two centers of circles when the fixation nut is tightened. In other words, when the fixation nut is tightened, the fixation element having the threaded portion and the fixation element having the slot form an angle which is fixed and cannot be changed. However, the angle can be adjusted slightly if the fixation nut is not tightened. It must be noted here that the fixation nut must be tightened so as to enable the fixation device to work effectively. Furthermore, the disclosure of the U.S. Pat. No. 4,854,311 comprises a washer which has a specific angle and is located between the threaded fixation element and the fluted fixation element. However, an addition of the washer can often complicates the surgical process.